Method of eliminating uncontrolled voids in sheet adhesive layer

ABSTRACT

A preformed adhesive layer for joining components within integrated circuit packaging includes venting slots for controlling the size and location of voids within an assembled integrated circuit package. Air randomly entrapped between the surfaces of the adhesive layer and adjoining components during assembly will generally release into the venting slots during subsequent assembly and/or mounting steps performed at elevated temperatures, rather than creating internal pressures causing separation of package components or releasing into the encapsulant. Die delamination and encapsulant void problems occurring during reflow or other assembly and mounting processes as a result of entrapped air are avoided.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention is directed, in general, to integratedcircuit packaging and, more specifically, to integrated circuit packagesutilizing preformed adhesive layers for affix package components to eachother.

BACKGROUND OF THE INVENTION

[0002] Solder ball grid array (BGA) integrated circuit packages areemployed to encapsulate integrated circuit die 15 for mounting onprinted circuit boards (PCBs) within various electronic devices such astelecommunications equipment. During mounting, electrical connection topackaged integrated circuit is achieved by reflowing the solder ballswhile in contact with corresponding connection traces on the printedcircuit board onto which the package is being mounted. Optional thinmounting profiles, high package “pin” counts, and ease of mounting arefeatures which make ball grid array packages attractive for a variety ofapplications including general purpose processors, digital signalprocessors (DSPs), and the like.

[0003] For “dense” integrated circuits having large numbers of closelyspaced, small feature size devices, a metallic heat spreader is oftenutilized within ball grid array packages to dissipate heat generatedduring operation of the integrated circuit. As illustrated by theexploded view shown in FIG. 4, a ball grid array package 400 of the typedescribed typically includes a printed circuit board substrate 401having a cavity 402 therein, which is affixed to a metal heat spreader403 utilizing a preformed adhesive layer 404 having a hole correspondingis size and position with cavity 402. The integrated circuit die (notshown) is mounted within the cavity, affixed to heat spreader 403, withwire bonds (not shown) connecting bonds pads in the integrated circuitdie to bonding sites on the substrate 401.

[0004] A significant defect commonly found with packages of the typeshown is the presence of voids between the surfaces of adhesive layer404 and the surfaces of the components (substrate 401 and heat spreader403 in this case) being joined by adhesive layer 404. As the componentsare being affixed utilizing adhesive layer 404, air is often randomlyentrapped between the surfaces of the components and the surfaces ofadhesive layer 404. If not removed before the adhesive layer 404hardens, uncontrolled voids (pockets of trapped air) will be formed inthe finished package between the adhesive layer 404 and adjoiningcomponents. Such voids may be removed by high vacuum and high pressureenvironments commonly employed in production of printed wiring boards(PWBs), but requires heavy industrial equipment.

[0005] Two problems have been found to frequently arise during assemblyand mounting of the package 400 as a result of uncontrolled voids in theadhesive layer 404: die delamination and voids within the encapsulantfor the package 400. Die delamination is caused by the expansion of airentrapped between the adhesive layer and adjoining components, applyingan internal pressure to cause separation of the package components fromthe adhesive layer 404 when the packaged device is heated duringfabrication of the packaged device or subsequent mounting of thepackaged device in a system.

[0006] Voids in the package encapsulant are similarly caused by gradualrelease of entrapped air while the encapsulant is being cured (typicallyat temperatures of approximately 175° C.). If these voids occur at theouter surface of the encapsulant, the packaged device is naturallyrejected. On occasion, however, the air bubble does not reach thesurface of the encapsulant and remains inside, a “buried” void which canresult in damage to the die or to the bonding wires inside theencapsulant. In both case, encapsulant voids affect manufacturing yieldand long term reliability of the product.

[0007] There is, therefore, a need in the art for improving integratedcircuit packages utilizing preformed adhesive layers.

SUMMARY OF THE INVENTION

[0008] To address the above-discussed deficiencies of the prior art, itis a primary object of the present invention to provide, for use in anintegrated circuit package, a preformed adhesive layer for joiningcomponents within integrated circuit packaging which includes ventingslots for controlling the size and location of voids within an assembledintegrated circuit package. Air randomly entrapped between the surfacesof the adhesive layer and adjoining components during assembly willgenerally release into the venting slots during subsequent assemblyand/or mounting steps performed at elevated temperatures, rather thancreating internal pressures causing separation of package components orreleasing into the encapsulant. Die delamination and encapsulant voidproblems occurring during reflow or other assembly and mountingprocesses as a result of entrapped air are avoided.

[0009] The foregoing has outlined rather broadly the features andtechnical advantages of the present invention so that those skilled inthe art may better understand the detailed description of the inventionthat follows. Additional features and advantages of the invention willbe described hereinafter that form the subject of the claims of theinvention. Those skilled in the art will appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. Those skilled in the art willalso realize that such equivalent constructions do not depart from thespirit and scope of the invention in its broadest form.

[0010] Before undertaking the DETAILED DESCRIPTION OF THE INVENTIONbelow, it may be advantageous to set forth definitions of certain wordsor phrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or” is inclusive, meaning and/or; and the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like.Definitions for certain words and phrases are provided throughout thispatent document, and those of ordinary skill in the art will understandthat such definitions apply in many, if not most, instances to prior aswell as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings,wherein like numbers designate like objects, and in which:

[0012]FIG. 1 is a plan view of a preformed integrated circuit packagingadhesive layer with venting slots according to one embodiment of thepresent invention;

[0013]FIG. 2 is a plan view of a sheet of preformed integrated circuitpackaging adhesive layers with venting slots according to one embodimentof the present invention;

[0014]FIG. 3 depicts a portion of an integrated circuit packageincluding a preformed adhesive layer with venting slots according to oneembodiment of the present invention; and

[0015]FIG. 4 depicts a portion of an integrated circuit packageincluding a preformed adhesive layer which suffers from a high incidenceof uncontrolled voids between the adhesive layer and adjoiningcomponents.

DETAILED DESCRIPTION OF THE INVENTION

[0016]FIGS. 1 through 3, discussed below, and the various embodimentused to describe the principles of the present invention in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the invention. Those skilled in the artwill understand that the principles of the present invention may beimplemented in any suitably arranged device.

[0017]FIG. 1 is a plan view of a preformed integrated circuit packagingadhesive layer with venting slots according to one embodiment of thepresent invention. Adhesive layer 100 is formed of a carrier material ofthe type employed to form printed circuit boards, with an epoxy orpolyimide adhesive, such as a fiberglass carrier impregnated with epoxyor a polyimide carrier deposited with B-staged polyimide. Adhesive layer100 is approximately 5 mils thick in the exemplary embodiment, andincludes a cavity hole 101 in accordance with the known art.

[0018] Adhesive layer 100 in the present invention also includes ventingslots 102-109 which extend through the thickness of the adhesive layer100, and may be formed by either die punching or laser cutting of theadhesive layer 100. As illustrated, venting slots 102-109 in theexemplary embodiment extend within adhesive layer 100 from a regionproximate to a side or corner of cavity hole 101 to a correspondingperipheral side or corner of adhesive layer 100.

[0019] Venting slots 102-109 serve as conduits for air and evaporated(gaseous) moisture within adhesive layer 100, which has been determinedto cause the randomly occurring voids described above. Venting slots102-109, approximately 1.5 mm wide in the exemplary embodiment, providechannels for conducting such moisture away from cavity region 101towards the periphery of the package. The air and moisture vapor may bedrawn out of the assembled package under the influence of a vacuumduring the adhesive curing process and/or expelled by the heatingrequired to reflow the solder balls during mounting of the packageddevice in a system.

[0020] Accordingly, bridge portions 110 of the adhesive layer 100,located at the terminating ends of venting slots 102-109 at theperiphery of adhesive layer 100 and separating the venting slots 102-109from the ambient environment outside adhesive layer 100, aresufficiently narrow to permit diffusion of air and moisture vaportherethrough while providing structural support and integrity foradhesive layer 100.

[0021] In the example shown, venting slots 102-109 include extensions111 around the periphery of cavity hole 101 and, in aggregate, extendsubstantially completely around cavity hole 101 to capture moisturediffusing out of adhesive layer 100 towards cavity hole 101. Portions112 of adhesive layer 100 separating venting slots 102-109 arepreferably wide enough to minimize diffusion of air or moisture vaporfrom venting slots 102-109 into cavity hole 101.

[0022]FIG. 2 is a plan view of a sheet of preformed integrated circuitpackaging adhesive layers with venting slots according to one embodimentof the present invention. Adhesive layer 100 may be produced in quantityby replicating the pattern of the cavity hole and venting slots for anumber of individual layers several times within a single sheet, theneither separating the individual layers (e.g., by laser cutting) orperforming a mass lamination utilizing a large sheet of heat sinkmaterial and a panel of the substrates with the sheet of preformedintegrated circuit packaging adhesive layers, singulating the individualpackages after lamination by cutting (saw or laser).

[0023]FIG. 3 depicts a portion of an integrated circuit packageincluding a preformed adhesive layer with venting slots according to oneembodiment of the present invention. Integrated circuit package 300includes a substrate 301 (e.g., a printed circuit board substrate)including a die cavity 302 extending therethrough. Substrate 301 isaffixed to another package component 303 using a preformed adhesivelayer 100 of the type described above. Package component 303 is ametallic heat spreader in the example shown, but alternatively may beanother substrate layer or any other package component.

[0024] During assembly, the epoxy resin with which adhesive layer 100 isimpregnated is cured to affix the substrate 301 to package component303. An integrated circuit die is mounted within die cavity 302 andaffixed, through cavity hole 101 within adhesive layer 100, to thesurface of package component 303 using conventional methods such asresin, etc. Wire bonds are then formed connecting the integrated circuitdie to conductive traces on substrate 301 which connect to solder ballswithin a ball grid array formed on substrate 301. The assembled packageis mounted by reflowing the solder balls to form electrical connectionsbetween the package and the structure on which the package is beingmounted.

[0025] Although the present invention has been described in detail,those skilled in the art will understand that various changes,substitutions and alterations herein may be made without departing fromthe spirit and scope of the invention it its broadest form.

What is claimed is:
 1. For use in an integrated circuit package, anadhesive layer for affixing one package component to another comprising:an adhesive material region; and at least one venting slot extendingwithin the adhesive material region for controlling a size and locationof voids between said adhesive material region and components joined bysaid adhesive material region during packaging.
 2. The adhesive layer asset forth in claim 1 wherein said at least one venting slot furthercomprises: a plurality of venting slots each extending from a regionproximate to a side or corner of a cavity hole within said adhesivematerial region towards a corresponding side or corner of a periphery ofsaid adhesive material region.
 3. The adhesive layer as set forth inclaim 2 wherein each said venting slot is separated from said peripheryof said adhesive material region by a width of adhesive materialsufficiently narrow to permit diffusion of gas therethrough.
 4. Theadhesive layer as set forth in claim 3 wherein each said venting slotfurther comprises: extensions extending along a periphery of said cavityhole.
 5. The adhesive layer as set forth in claim 4 wherein saidextensions are spaced from said cavity hole by adhesive materialsufficiently wide to minimize diffusion of gas from said venting slotsinto said cavity hole.
 6. The adhesive layer as set forth in claim 1wherein said adhesive material region is formed of fiberglassimpregnated with epoxy resin.
 7. The adhesive layer as set forth inclaim 1 wherein said adhesive material region is formed of polyimidefilm with B-staged polyimide adhesive resin.
 8. The adhesive layer asset forth in claim 1 wherein said adhesive layer comprises a sheetforming a plurality of said adhesive material regions, each adhesivematerial region including said at least one venting slot.
 9. Anintegrated circuit package comprising: a first package component; asecond package component; and an adhesive layer affixing said first andsecond package components to each other, said adhesive layer comprising:an adhesive material region; and at least one venting slot extendingwithin the adhesive material region for controlling a size and locationof voids between said adhesive layer and said first or second componentduring packaging.
 10. The integrated circuit package as set forth inclaim 9 wherein said at least one venting slot further comprises: aplurality of venting slots each extending from a region proximate to aside or corner of a cavity hole within said adhesive material regiontowards a corresponding side or corner of a periphery of said adhesivematerial region.
 11. The integrated circuit package as set forth inclaim 10 wherein each said venting slot is separated from said peripheryof said adhesive material region by a width of adhesive materialsufficiently narrow to permit diffusion of gas therethrough.
 12. Theintegrated circuit package as set forth in claim 11 wherein each saidventing slot further comprises: extensions extending along a peripheryof said cavity hole.
 13. The integrated circuit package as set forth inclaim 12, wherein said extensions are spaced from said cavity hole byadhesive material sufficiently wide to minimize diffusion of gas fromsaid venting slots into said cavity hole.
 14. The integrated circuitpackage as set forth in claim 9, wherein said adhesive material regionis formed of fiberglass impregnated with epoxy resin.
 15. The integratedcircuit package as set forth in claim 9, wherein said adhesive materialregion is formed of polyimide film with B-staged polyimide adhesiveresin.
 16. The integrated circuit package as set forth in claim 9further comprising: an integrated circuit die mounted within a diecavity within said first package component and affixed through a cavityhole within said adhesive material region to said second packagecomponent.
 17. For use in an integrated circuit package, a method ofaffixing one package component to another comprising: forming anadhesive material region; and forming at least one venting slotextending within the adhesive material region for controlling a size andlocation of voids between said adhesive material region and componentsjoined by said adhesive layer during packaging.
 18. The method as setforth in claim 17 wherein the step of forming at least one venting slotextending within the adhesive material region for controlling a size andlocation of voids between said adhesive material region and componentsjoined by said adhesive material region during packaging furthercomprises: forming a plurality of venting slots each extending from aregion proximate to a side or corner of a cavity hole within theadhesive material region towards a corresponding side or corner of aperiphery of the adhesive material region.
 19. The method as set forthin claim 18 wherein the step of forming a plurality of venting slotseach extending from a region proximate to a side or corner of a cavityhole within the adhesive material region towards a corresponding side orcorner of a periphery of the adhesive material region further comprises:forming each venting slot to extend to a point separated from theperiphery of the adhesive material region by a width of adhesivematerial sufficiently narrow to permit diffusion of gas therethrough;and forming extensions of each venting slot extending along a peripheryof the cavity hole, wherein the extensions are spaced from the cavityhole by adhesive material sufficiently wide to minimize diffusion of gasfrom the venting slots into the cavity hole.
 20. The method as set forthin claim 17 further comprising: forming a plurality of patterns for theadhesive material region within a sheet, each pattern including the atleast one venting slot.
 21. The method as set forth in claim 17 furthercomprising: affixing a substrate having a die cavity therein to a secondpackage component utilizing the adhesive material region.
 22. The methodas set forth in claim 21 further comprising: mounting an integratedcircuit die within the die cavity; affixing the integrated circuit dieto the second package component through the cavity hole; andelectrically connecting the integrated circuit die to conductive traceson the substrate.